This invention relates to the field of parameter estimation using correlation analysis, and more specifically, to a parameter estimator employing correlation analysis for estimating parameter(s) of signals subject to distortions caused by factors such as multi-path.
The Global Positioning System (GPS) is a collection of satellites each of which travels in a precise orbit above the earth""s surface. Each satellite transmits a signal modulated with a pseudo-noise (PN) code unique to the satellite. Each PN code comprises a predetermined number of chips. A GPS receiver receives a composite signal comprising a mixture of signals from each of the satellites that are visible to the receiver. A signal estimator in the receiver detects a transmission from a particular satellite by determining the degree of correlation between the received signal and shifted versions of the PN code for that satellite. If a peak of sufficient quality in the correlation value for one of the shift offsets is detected, the receiver is considered to have detected the transmission from the satellite.
The receiver estimates its location by detecting transmissions from at least four of the satellites. For each detected transmission, the receiver uses the shift in the PN code to estimate the delay (in terms of chips or fractions of chips) between time of transmission and time of arrival. Given the known velocity of the transmission, the receiver estimates the distance between itself and the satellite. This estimated distance defines a sphere around the satellite. The receiver knows the precise orbits and positions of each of the satellites, and continuously receives updates to these orbits and positions. From this information, the receiver is able to estimate its position (and the current time) from the point where the spheres for the four satellites intersect.
The FCC has mandated that subscriber stations, including but not limited to mobile stations, in wireless communications systems be capable of estimating their locations in order to promote rapid responses to 911 and other emergency calls. In response to this mandate, efforts are underway to equip subscriber stations with the means to estimate their locations from GPS satellite transmissions. Moreover, since base stations in wireless communications systems transmit pilot signals modulated with unique PN codes, these efforts also include allowing subscriber stations to estimate their locations from the transmissions of multiple base stations or sectors, or combinations of base stations or sectors and GPS satellites.
A signal detector in a GPS receiver attempts to detect the transmission from a satellite from a correlation function which is derived by multiplying the received signal (which is typically a composite signal comprising a mixture of the transmissions from multiple satellites) with shifted versions of the PN code for the satellite within a range defined by a predetermined search window, and then, for each shifted PN code, adding the multiplied values over a predetermined integration time to achieve a value representative of the degree of correlation between the received signal and the shifted PN code.
However, such a detector is not generally effective for the purpose of detecting transmissions from multiple base stations or sectors since, unlike the search windows used for searching for GPS satellites, the search windows used for searching for base stations or sectors are determined by the network and are not optimized for position location technologies. The search windows provided by the network are optimized for handoff performance. Consequently, the estimator will typically exceed available search time constraints if the size of the predetermined search window is set too large or the window is inappropriately placed.
Consider, for example, a detector which attempts to detect 40 different base station or sector signals using a predetermined search window of 400 different PN code offsets for each base station or sector signal. If the detector employs a relatively long integration time, e.g., 26.67 mS, so it can detect the weakest signals, even assuming it can evaluate 16 offsets simultaneously, the detector will require 26.67 seconds to perform the search, which is prohibitive given what are typical time constraints of 2-4 seconds.
This application is related to U.S. patent application Ser. No. 10/057,689 filed on even date herewith, Ser. No. 10/060,885, filed Jan. 29, 2002, and Ser. No. 10/125,182, filed Apr. 17, 2002, all owned in common by the assignee hereof.
The invention provides a parameter estimator for estimating one or more parameter(s) of a signal through correlation analysis using a dynamically variable search window. For purposes of this disclosure, a dynamically variable search window is one where the size and/or placement of the window can vary responsive to a priori information regarding the signal, or can vary from signal to signal, or can vary from estimation attempt to estimation attempt, or can vary from default values, or any combination of the foregoing. The signal may be a standalone signal or part of a composite signal comprising multiple signals. Examples of the parameter(s) which may be estimated include, but are not limited to, time of arrival (TOA), root mean squared error (RMSE) for the TOA estimate, energy per chip (Ec) divided by interference noise density (I0), etc.
The estimator comprises correlation logic and analysis logic. The correlation logic determines a correlation function of a signal in relation to a selected identification code which, in one embodiment, is a PN code. The correlation function represents the correlation between the signal and shifted versions of the identification code. The range of the shifted versions of the identification code which are considered defines a search window. The size and/or placement of the search window can dynamically vary responsive to a priori information regarding the signal, or from signal to signal, or from estimation attempt to estimation attempt, or from default values, or combinations of the foregoing. The analysis logic analyzes the correlation function for the signal and, responsive thereto, determines one or more parameter(s) for the signal.
Various methods of operating the estimator are possible. In one embodiment, the estimator is provided with a priori information regarding a signal, and, responsive thereto, dynamically determines the size and/or placement of a search window. In one implementation, the estimator may employ a default search window and dynamically vary the size and/or placement of the search window from the default values responsive to a priori information regarding the signal which is provided to it. It then derives from the signal a correlation function representing the correlation between the signal and shifted versions of an identification code, where the range of the shifted identification codes which are represented is defined by the search window. It then attempts to estimate one or more parameter(s) relating to the signal from the correlation function.
The a priori information may be provided from a source external to the estimator, e.g., a wireless communications system in which a subscriber station including the estimator is employed. Alternatively, or in addition, the a priori information may be provided from the estimator itself, e.g., from a previous search performed in relation to the signal. Alternatively, or in addition, the a priori information may consist of a general knowledge of channel characteristics, e.g., weaker signals typically have later time of arrivals than the strongest signals.
When the technique represented by any of these methods is applied to a group of signals during a search cycle, performance is improved compared to the performance using the default search window parameters provided by the network, since the size and/or placement of the search window which is used may be tailored to an individual signal based on a priori information regarding the signal. Therefore, search cycle time is reduced.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.